Depends on my mood, but it's become a tradition for my dad, my brother and I to have a Toffee Crisp at half time while watching Town. Seems to be helping us on the pitch too this season, in some unfathomable way!
As I have type-1 diabetes I don't indulge often, but when I do, you can't beat a Boost bar. Related to my condition I usually have at least one fun-size bag of Skittles on my person for emergencies.
A slightly odd occasional treat is to buy a "share-bag" size bag of Revels (and yes, I do love the coffee and orange ones too) and leave it in the car for a couple of hours on a hot day. Then place it in the fridge to harden before attacking it with a toffee hammer. Maybe I'm just weird?
A special mention for Dairy Milk Oreo bars, and for Reese's Peanut Butter Cups which are the only American chocolate that doesn't taste of vomit (looking at YOU, Hershey bars!).